Use of handheld X-ray fluorescence as a non-invasive method to distinguish between Asian and African elephant tusks

Abstract

We describe the use of handheld X-ray fluorescence, for elephant tusk species identification. Asian (n = 72) and African (n = 85) elephant tusks were scanned and we utilized the species differences in elemental composition to develop a functional model differentiating between species with high precision. Spatially, the majority of measured elements (n = 26) exhibited a homogeneous distribution in cross-section, but a more heterologous pattern in the longitudinal direction. Twenty-one of twenty four elements differed between Asian and African samples. Data were subjected to hierarchical cluster analysis followed by a stepwise discriminant analysis, which identified elements for the functional equation. The best equation consisted of ratios of Si, S, Cl, Ti, Mn, Ag, Sb and W, with Zr as the denominator. Next, Bayesian binary regression model analysis was conducted to predict the probability that a tusk would be of African origin. A cut-off value was established to improve discrimination. This Bayesian hybrid classification model was then validated by scanning an additional 30 Asian and 41 African tusks, which showed high accuracy (94%) and precision (95%) rates. We conclude that handheld XRF is an accurate, non-invasive method to discriminate origin of elephant tusks provides rapid results applicable to use in the field.

Figure 1

Schematics illustrate longitudinal (A) and transversal (B) XRF scanning sites of the enamel layer on elephant tusks for Asian and African elephants combined. Comparison of elemental levels (ppm) among longitudinal and transversal sites, and coefficients of variation (CV) are shown in (C). Significant p values (<0.05) are highlighted in blue, with concentration differences highlighted by the level of red intensity. (Mg = magnesium, Al = aluminum, Si = silicon, P = phosphorous, S = sulfur, Cl = chlorine, K = potassium, Ca = calcium, Ti = titanium, V = vanadium, Cr = chromium, Mn = manganese, Fe = iron, Co = cobalt, Ni = nickel, Cu = copper, Zn = zinc, As = arsenic, Zr = zirconium, Ag = silver, Cd = cadmium, Sn = tin, Sb = antimony, W = tungsten, Pb = lead, LE = light element).

Figure 2

Differences in elemental composition between genders (Asian only, A), cross-sectional layers (B) and species (C). Coefficients of variation (CV), with darker red boxes indicating higher CVs. Elements in the blue boxes are those used as a denominator in creating the discriminant equation (D). (Mg = magnesium, Al = aluminum, Si = silicon, P = phosphorous, S = sulfur, Cl = chlorine, K = potassium, Ca = calcium, Ti = titanium, V = vanadium, Cr = chromium, Mn = manganese, Fe = iron, Ni = nickel, Cu = copper, Zn = zinc, Zr = zirconium, Ag = silver, Cd = cadmium, Sn = tin, Sb = antimony, W = tungsten, Pb = lead, LE = light element).

Figure 3. Pearson’s correlation coefficients between each element for Asian and African elephant tusks combined, as determined by XRF analysis.

Colored boxes indicate direction and strength of each correlation. (Mg = magnesium, Al = aluminum, Si = silicon, P = phosphorous, S = sulfur, Cl = chlorine, K = potassium, Ca = calcium, Ti = titanium, V = vanadium, Cr = chromium, Mn = manganese, Fe = iron, Co = cobalt, Ni = nickel, Cu = copper, Zn = zinc, Zr = zirconium, Mo = molybdenum, Ag = silver, Cd = cadmium, Sn = tin, Sb = antimony, W = tungsten, Pb = lead).

Figure 4. Hierarchical cluster analysis of individual elemental compositions obtained from XRF scanning of Asian and African tusks.

Figure 5. Effectiveness of the elemental composition data for estimating origin of elephant tusks.

(A) Scatter plot and density of values using discriminant analysis with Zr as the denominator based on scans of 72 Asian and 85 African tusks. (B) Predicting Asian or African tusk origins (Asian = 0, African = 1) as a function of the discriminant values (DV) subjected to Bayesian logit regression, referred as to a Bayesian hybrid classification model. Data of DV are indicated by the dots according to tusk species. (C) The set of cut-off values for predicting Asian or African tusk origin. Dashed red line shows the level of kappa at 0.8. (D) Scatter plot testing the reliability of the model for estimating tusk origin for 30 Asian and 41 African tusks with a cut-off value of 55% (dashed red line).

Figure 6. Phylogenetic tree based on cytochrome b sequence.